1. Field of the Invention
The present invention relates to a dialkyl ether, and to a lubricating base oil and a lubricating oil composition containing the dialkyl ether.
2. Background Art
In recent years, CO2 emission has been required to be reduced more and more from the viewpoint of protection of the global environment. Under such circumstances, there is demand for a drastic improvement in fuel-saving performance of lubricating oil. For attaining fuel savings, reduction of viscous resistance is an effective measure, and a variety of low-viscosity lubricating base oils have been developed. Hitherto, (1) ester, (2) poly-α-olefin, (3) silicone oil, and the like have been developed as low-viscosity lubricating base oils. However, these lubricating base oils have the following problems.
Specifically, when used in an engine oil, ester (1), having high polarity, adversely affects organic parts, resulting in swelling of rubber sealing parts. Poly-α-olefin (2) does not affect organic parts, but difficulty is encountered in establishing suitable balance between viscosity index and pour point (flowability at low temperature), when the viscosity thereof is reduced. Silicone oil (3) has low viscosity and high viscosity index, and exhibits excellent flowability at low temperature. However, silicon oil has problems including poor lubricity and poor compatibility with existing additives.
In order to solve these problems, there has recently been developed a lubricating base oil which contains a compound having an ether bond and which exhibits a kinematic viscosity at 40° C. and an aniline point falling within specific ranges (see Patent Document 1).
It is true that a lubricating base oil containing a compound having an ether bond has low viscosity and high viscosity index, exhibits excellent flowability at low temperature, and is highly compatible to organic parts, thereby enabling use in a lubricating oil such as an engine oil for an internal combustion engine or the like. However, in order to meet needs for further improved fuel-saving performance, such a lubricating base oil must attain a further reduced viscosity and well-balanced viscosity-temperature characteristics and flowability at low temperature.
Specifically, the lubricating base oil disclosed in Patent Document 1 exhibits a kinematic viscosity, as measured at 100° C., of 3.2 to 2.4 mm2/s. In this case, the lower the viscosity, the lower the viscosity index. Currently, there has not been clearly elucidated the structure of the ether compound which enables to attain well-balanced viscosity index and low-temperature flowability of the base oil when the viscosity thereof is further reduced. Patent Documents 2 and 3 disclose a lubricating base oil containing a monoether. However, the disclosed alkyl group combinations at both ends are limited. Patent Documents 4 to 7 and Non-Patent Document 1 disclose monoether compounds, but do not disclose a monoether that can improve viscosity index and flowability at low temperature or a lubricating base oil containing the monoether.    Patent Document 1: WO 2004/058928    Patent Document 2: WO 2004/090082    Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. 2005-344017    Patent Document 4: Japanese Patent Publication (kokoku) No. 1974-12746    Patent Document 5: Japanese Patent Application Laid-Open (kokai) No. 1973-039507    Patent Document 6: Japanese Patent Application Laid-Open (kokai) No. 1982-117596    Patent Document 7: Japanese Patent Application Laid-Open (kokai) No. 1997-087223    Non-Patent Document 1: Wilhelm Schlenk Jr., “Konfigurativ Stetige Gitterzuordnung der Gastmolekule,” Liebigs Ann. Chem. 1973, 1156-1178